The catalytic production of dimethyl ether (DME) from methanol by catalytic dehydration has been known for many years. The U.S. Pat. No. 2,014,408 for example describes a process for the production of DME from methanol on catalysts such as aluminum oxide, titanium oxide and barium oxide, with temperatures of 350 to 400° C. being preferred.
Further information on the prior art and on the current practice of the production of dimethyl ether can be found in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “dimethyl ether”. In Chapter 3 “Production” it is explained in particular that the catalytic conversion of pure, gaseous methanol is performed in a fixed-bed reactor.
From the point of view of reaction engineering, fixed-bed reactors preferably are used for the catalytic dehydration of methanol to DME in the gas phase, since they are characterized by constructive simplicity. The German laid-open publication DE 3817816 describes a process integrated in a methanol synthesis plant for producing dimethyl ether by catalytic dehydration of methanol without previous separation of the synthesis gas not converted in the methanol reactor. As dehydration reactor a simple fixed-bed reactor is used. When the same is designed for temperature control without any additional measures, but is merely surrounded with an outer insulation to avoid heat losses, it is also referred to as adiabatic fixed-bed reactor.
The dehydration of methanol to dimethyl ether according to the reaction equation2CH3OH═(CH3)2O+H2Ois an exothermal equilibrium reaction; hence it follows that from a thermodynamic point of view high degrees of conversion are achieved at reaction temperatures as low as possible. On the other hand, from a reaction-kinetic point of view a minimum reaction temperature is required, in order to ensure sufficient reaction rates and thus acceptable methanol conversions. What is disadvantageous in the adiabatic fixed-bed reactors used so far is the missing possibility of ensuring an optimum temperature control, to ensure high degrees of conversion and to minimize the formation of by-products.
The formation of by-products, such as carbon monoxide CO, carbon dioxide CO2, hydrogen H2 and methane CH4, preferably is effected at higher temperatures. As a possible cause for the formation of the three first-mentioned by-products, the steam cracking of methanol in the feed stream or of already formed DME with steam is assumed, which steam is formed as reaction by-product. Methane for example can be formed as a consecutive reaction of the formed carbon oxides with hydrogen. The formation of these by-products is undesirable, since they impair the purity of the reaction product and reduce the selectivity of the reaction to DME.
The theoretical study “Modeling and Optimization of MeOH to DME Isothermal Fixed-bed Reactor”, Farsi et al., International Journal of Chemical Reactor Engineering, Volume 8, 2010, Article A79, describes the optimum temperature profile of the reactor temperature for the catalytic dehydration of methanol to dimethyl ether in a quasi (or largely) isothermal fixed-bed reactor, in which the solid catalyst is arranged in tubes which on the shell side are surrounded by partly evaporating water as cooling medium. By using a genetic algorithm which takes account of the thermodynamic and kinetic aspects of the dehydration reaction, a temperature profile exponentially decreasing from the reactor inlet to the reactor outlet is calculated as optimum, wherein the reactor inlet temperature is about 800 K and the reactor outlet temperature is about 560 K. Proceeding from this axial temperature profile, a methanol conversion of about 86% is calculated for the optimized isothermal reactor, whereas it is merely about 82% in the adiabatic reactor. However, said paper does not provide any information as to how an optimized fixed-bed reactor should constructively be designed for the production of DME from methanol. Moreover, merely the methanol conversion, but not the formation of possible by-products is used as criterion for optimization.